Background: Changes in mood on occasions of clinical severity were reported in late-stage Parkinson's Disease (PD) patients who underwent subthalamic deep brain stimulation (STN-DBS) (Berney et al. 2002, Anderson et al. 2003; for review). The aim of this study was to further characterize long-term mood outcome in PD patients prospectively studied for at least 2 years after STN-DBS implantation.Method: PD patients were systematically examined preoperatively (T0), at 10.2F2.9 (T1) and 35.7F10.6 months (T2) following STN-DBS, using the Montgomery Asberg Depression Rating Scale (MADRS), and a brief structured psychiatric interview (MINI), in addition to the cognitive and motor assessments.Results: Fifty PD patients (30 male and 20 female; mean PD duration, 15.1F4.8 years; mean age at implantation, 64.7F7.9 years) were studied. Repeated measures analysis of variance on MADRS scores showed a significant main effect of time; post hoc analysis revealed that patients significantly worsened their mood scores from baseline to T1 (P=.03), whereas mood scores did not significantly change from T1 to T2. Using Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition diagnostic criteria for major depression and a conservative cutoff score of 20 on the MADRS, (1/50) 2% were depressed at baseline, (8/50) 16% at T1 and (5/43) 12% at T2. Female patients appeared to be at higher risk for developing major depression in our sample.Conclusions: This long-term follow-up study reveals a high rate of Major Depressive Episodes occurring during the first year following DBS neurosurgery for PD. STN-DBS may precipitate the development of comorbid depression in at-risk individuals.